The invention relates to a process for producing a photosensitive coating; in particular it relates to a process for producing a printing plate comprising such a coating as well as printing plates obtained by the method.
In recent times, the requirements with regard to the print run stability of offset printing plates have increased. For the improvement of the mechanical properties of offset plates, basically two methods have been discussed.
One of them relates to the improvement of the properties of the binders or the photosensitive composition (U.S. Pat. No. 4,102,686). This method often entails high costs due to the complexity of the synthesis regarding the preparation of such custom high-performance binders.
The other, less expensive method is the introduction of polymer particles into the photosensitive layer. The mechanical resistance of such particle-containing layers is often superior to that of layers with modified binders. Apart from the increase of the mechanical resistance of the layer, there are further advantages: From the literature it is known that the chemical resistance can be increased while the exhausting times in the copying frame and the adhesiveness can be reduced. So far, various methods have been employed for introducing the polymer particles into the layer.
Commercially available micronized particles which are difficult to dissolve in common organic solvents (for example polyethylene, polytetrafluoroethylene, cross-linked polymethylmethacrylate or polyamide) are dispersed and added to the coating solution. This entails the disadvantage that these polymer particles first have to be dispersed which constitutes an additional process step. In some areas, the particles are often distributed rather inhomogeneously after the coating has been completed. Furthermore, in particles consisting of nonpolar polymers the adhesion to the layer is reduced. The retention time of the coating solution prepared is limited as well since these dispersions tend to form sediments or agglomerates. Furthermore, the particle dispersions cause problems in the filtering of the coating solution since the pore size of the filter cannot be smaller than the particle diameter. Starting solutions which have not been sufficiently filtered often lead to coating defects.
Another method is to prepare dispersions or emulsions of polymers by emulsion/dispersion polymerization, for example in copolymers of acrylates, styrene derivatives and other monomers, or by precipitation reaction of polymers which dissolve in common organic solvents, for example acrylonitrile butadiene styrenes (ABS) which are then added to the coating solution (U.S. Pat. No. 4,233,390, EP-A-0 709 399, U.S. Pat. No. 5,616,449, EP-A-0 632 328, EP-B-0 509 523).
Thus, due to the higher polarity of the polymer particles, an improved adhesion of the particles to the layer is achieved, but the above-mentioned disadvantages such as poor filterability, locally inhomogeneous distribution in the layer, and short stability of the batch remain.
Another process describes, with regard to positive working printing plates, the use of specifically developed polymers which are soluble in alkaline developers and organic media and which form a homogenous solution with the coating composition. By way of complicated drying apparatuses difficult to operate with regard to the process control, a separation of the binders intended for the positive plates and the polymers during drying is achieved (EP-A-0 766 140). This coating solution can be filtered since a xe2x80x9crealxe2x80x9d solution is present. A disadvantage of this method is that the particles are preferably formed in the upper part of the photosensitive layer, with the lower part of the layer not being protected from mechanical abrasion. Furthermore, both the drying apparatus and the polymers which form the particles, have to be specifically adapted to this purpose of use, which constitutes a high cost factor. A further disadvantage of this method is that the resulting polymer particles, which are soluble in the developer, can partly be extracted from the layer by the developing step and that the remaining cavities rather destabilize the image areas.
In U.S. Pat. No. 6,238,831 based on PCT/DE99/016730 coating solutions are disclosed which comprise:
a) a positive or negative working, or electrophotographically working photosensitive composition,
b) a thermoplastic polymer which is soluble in organic media but insoluble in alkaline media,
c) a solvent component A which dissolves both the photosensitive composition and the thermoplastic polymer, and optionally
d) a solvent component B which dissolves the photosensitive composition but not the thermoplastic polymer and which is less volatile than component A,
with a) and b) being homogeneously dissolved in the mixture of c) and d).
The offset printing plates obtained according to this application show high print run stability, are developable by means of conventional developers, exhibit high resolutions, have sufficient resistance to common cleaning chemicals and show fast and good ink resistance. However, there is a need for improving the method for producing such plates as disclosed in U.S. Pat. No. 6,238,831 since the line speed cannot be increased over 20 m/min because the particles need some time to grow.
It is therefore the object of the present invention to provide a method for producing offset printing plates having high print run stability which enables the production of printing plates with particle-containing photosensitive layers at high line speeds, i.e. a line speed over 20 m/min.
This object is achieved by a method comprising the following steps:
(a) providing a first solution comprising a thermoplastic polymer P and a solvent component A, the thermoplastic polymer being soluble in organic media but insoluble in alkaline media and solvent A being capable of dissolving the thermoplastic polymer and a conventional photosensitive composition
(b) separately providing a second solution comprising a solvent component B which is capable of dissolving a conventional photosensitive composition but not the thermoplastic polymer and which is less volatile than solvent A
(c) separately feeding the solutions of steps (a) and (b) above to an in-line mixer unit
(d) directly transferring the obtained mixture to a coating unit
(e) applying the mixture to a printing plate substrate
wherein a conventional positive or negative working, or electrophotographically working photosensitive composition C is either present in one of the first and second solution or in both solutions.